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Q & A: How do we define communication?

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Most recent answer: 09/12/2016
Why we say that information can't be shared using entangled particles or that events based on entanglement are not communication of information above speed of light.Let's say there are two entangled particles A and B separated by multiple light years and located-in and observed by lab-A and lab-B (entangled on Spin or anything else). Now these two labs can at least communicate what is happening to that particle in the other lab without any classical communication and this knowledge is instantaneous. while what is happening is random (spin of particle at either location, so not very meaningful) but what is shared (the observation of spin) is real communication?We should say that in this case communication is faster than light for sure but what gets communicated is random happening. In the physical communication, we are actually relying not on communication but teleporting of material itself, say electrons through optic fibre or radio waves moving through a medium etc. Is our idea of what communication of information means is slightly confusing?
- shashikant (age 44)
redwood city, ca, usa

Usually when we say that we can't communicate faster than the speed of light using entangled particles, we mean you can't use them to send a message. As you correctly described, the outcomes of measurements on entangled particles are random, so you couldn't communicate anything by, for example, assigning them to mean "yes" or "no" (or 0 or 1). The reason we care about this is because according to special relativity, if you could send messages faster than light there would be no guaranteed causality—effects could happen before their causes in some reference frames.

It's true that you gain some instantaneous knowledge about the state of a particle in a far away lab as soon as you measure your particle, and you might call that communicating information. On the other hand, did that information really "travel" to you, or is it just a deduction based on your local measurement and your knowledge of quantum mechanics? I'm leaning towards the latter. 

Rebecca H.

(published on 09/12/2016)

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